Generally, die castings are produced by forcing a molten metal under pressure into a steel die and maintaining the molten metal under pressure until solidification of the molten metal into a casting is complete. A wide variety of metal and metal alloys may be used in die casting processes. For example, aluminum alloys, brass alloys and zinc alloys are all commonly used in die casting processes to form die castings. Broadly speaking, a die casting process requires the following elements: (a) a die-casting machine to hold a molten metal or metal alloy under pressure; (b) a metallic mold or die capable of receiving the molten metal or metal alloy and designed to permit easy and economical ejection of the solidified metal or metal alloy die casting; and (c) a metal or metal alloy which, when solidified into a metal or metal alloy die casting, will produce a satisfactory product with suitable physical characteristics.
There are two types of die-casting machines commonly in use today. The first, or cold-chambered, die-casting machine forces the molten metal or metal alloy into the die by means of a plunger and chamber located outside the molten metal or metal alloy bath. Conversely, the second, or hot-chamber, die-casting machine forces the molten metal or metal alloy into the die by means of a plunger and chamber which are submerged in the molten metal or metal alloy bath. Depending on the production requirements therefore, the metallic mold or dies to be used in die casting processes may be constructed in different styles. A “single” die contains an impression of only one part; a “combination” die contains an impression of multiple parts; a “multiple” die contains two or more impressions of a single part; and a “combination-multiple” die contains a number of impressions of each one of two or more parts. Single dies are comparatively cheap and, since they reduce the tool investment to a minimum for any one part, are typically used for small lot productions. When properly designed, combination dies will reduce the total die cost for a given set of die castings to a minimum. They are particularly useful for die castings that will always be used in the same quantities and formed of the same alloy. Multiple dies are usually slower to operate than single dies but will give higher production rates for the same labor costs.
It should be readily appreciated that a wide variety of die castings may be produced by application of conventional die casting manufacturing principles. One such die casting is an aluminum alloy die casting. Similarly, while aluminum alloy die castings may be used in a wide variety of applications, in one such application, specially shaped aluminum alloy die castings are used as the rocker cover and the rocker housing for the FL Series motorcycle currently manufactured by the Harley-Davidson Motor Company of Milwaukee, Wis. To enhance the appearance thereof, prior to mounting of the rocker cover and rocker housing die castings on the FL Series motorcycle, the aluminum alloy die castings are plated with chromium. Traditionally, the aluminum alloy die castings have been manufactured at a first facility and subsequently shipped to a second facility for plating.
A drawback to this process has been that, once subjected to the chrome-plating process, the aluminum alloy die castings produced at the first facility often proved unsuitable for their intended later use. For example, using conventional die casting techniques, chrome-plated aluminum alloy die castings to be used as either a rocker cover or rocker housing for the aforementioned FL Series motorcycles were experiencing a rejection rate of about 40% due to defects noted during inspections of the die castings conducted during and/or after the chrome-plating process. While the rejection rate has been attributed to a variety of causes, one such cause is that a number of the various types of defects which commonly occur during the manufacture of an aluminum alloy die casting can remain unnoticed until after an attempt has been made to chrome-plate the die casting.
It should be readily appreciated that a rejection rate of about 40% adds considerably to the cost of chrome-plated aluminum alloy rocker covers or chrome-plated aluminum alloy rocker housings. It should also be readily appreciated that substantial cost savings may be achieved by reducing the rejection rate of chrome-plated aluminum alloy rocker covers, chrome-plated aluminum alloy rocker housings and other products manufactured using die casting processes which are currently plagued by high rejection rates. Achieving a reduction in such rejection rates is, therefore, an object of the present invention.